JRuby represents a Ruby implementation which is better than the original - it allows you to use Java libraries in a Ruby application, and typically out-paces C Ruby in every respect. It also simplifies Rails deployment (no need for running multiple instances of a server, as it can run inside a J2EE container).
The only problem I had was with database access - the adapters aren't quite up to scratch with the C Ruby libraries, but I expect this to stabilise.
Java integration is really good - you can configure the runtime such that limited permission environments are easy to cater for - the BrowserScripting project illustrates this by allowing Ruby scripting from within an applet (probably the most permission constrained environment you can have for Java).
I would give this a 5/5 rating, except for my personal dislike for the Ruby language itself.